Here, ripples on the water surface act as lenses to direct converging
cones of light downwards. When water is slightly turbid they become
visible but not as you might expect. The light cones are all parallel
to each other but perspective effects make them appear to radiate outwards
from the point directly opposite the sun where the shadow of your head
is located.

Where the focused light shines on the streambed it forms a moving
pattern of bright curves - caustics. Watch them carefully to see gem-like
flashes of colour as the water lenses re-image them.